A) Field of the Invention
This invention relates to a liquid crystal display.
B) Description of the Related Art
When no voltage is applied, a direction of a long axis of nematic liquid crystal is oriented in a plane parallel to a substrate surface with which a liquid crystal layer is contacted. The direction of a long axis of nematic liquid crystal can be oriented in parallel to a rubbing direction by forming an orientation film on the substrate surface and thereon forming an orientation structure by a rubbing process. Incident light can be considered by classifying into a polarization component along with a long axis of the liquid crystal molecules and a polarization component along with a short axis perpendicular to the long axis.
An optical rotating structure wherein the liquid crystal molecules twist at a particular angle in a thickness direction can be realized by using a pair of opposing substrate with different orientations. When irradiating linearly polarized light along with the orientation of the liquid crystal molecules at a light incident surface of the liquid crystal layer, a polarization direction of the incident light can be rotated along with the twist of the liquid crystal molecules and output linearly polarized light rotated at the twist angle. When a threshold voltage or a voltage above a threshold value (ON-voltage) is applied between the opposing substrates, a long axis of liquid crystal molecule stands up and the optical rotation power will be lost. Depending on the existence of the optical rotation power, ON/OFF state of display can be controlled. The optical rotation power can be selectively controlled by forming electrodes in selected shapes on the opposing substrates and controlling voltages between the electrodes. The existence of the optical rotation power can be detected by a pair of polarizers, a polarizer and an analyzer. A nematic liquid crystal display with the twist angle of 90 degrees is called a twisted nematic (TN) liquid crystal display.
A polarizer having a polarization axis parallel to orientation is placed at a light incident side of a liquid crystal layer with a twist angle of 90 degrees, and an analyzer having a polarization axis parallel or perpendicular to a polarization direction rotating at 90 degrees is placed at a light output side of the liquid crystal layer. If a polarizer having a polarization direction parallel to a polarization direction of passing light is placed at the light output side of the liquid crystal layer (arranged in a crossed Nicols state), it will be a normally white display. If a polarizer having a polarization direction perpendicular to a polarization direction of passing light is placed at the light output side of the liquid crystal layer (arranged in a parallel Nicols state), it will be a normally black display. A nematic liquid crystal display with a twist angle over 90 degrees is also possible. A nematic liquid crystal display with a twist angle over 90 degrees is called a super twisted nematic (STN) liquid crystal display.
In a simple matrix driving method, for example, common electrodes arranged in parallel to each another in an x-direction and segment electrodes arranged in parallel to each another in an y-direction are opposed to each another to display a pixel at selected x and y addresses. The common electrodes are scanned, and image signals corresponding to a display image are applied to the segment electrodes. The image signals not less than the threshold value (ON-voltage) are applied to the selected pixels. A non-selection voltage less than the threshold value is applied to unselected pixels. No voltage is applied to a background region where no opposing electrode exists. The non-selection voltage is set to within a range which can maintain a state of orientation of the liquid crystal molecules similar to a state when no voltage is applied. In the simple matrix driving method, it is necessary to increase number of pixels to improve resolution. It is necessary to increase number of scanning lines (increase a duty ratio), and it is desired for pixels to response fast and with high precision toward application of a threshold voltage or a voltage over a threshold value. A TN type liquid crystal display has a limit on a response speed, and so it is not suitable for a high-duty ratio operation. An STN type liquid crystal display is more suitable for a high-duty ratio operation.
Generally a STN type liquid crystal display shows a transmission spectrum having a maximum value at a certain wavelength and a minimum value at a certain wavelength when no voltage is applied. The transmission spectrum when the selection voltage is applied is assumed to be basically the same as the transmission spectrum when no voltage is applied. There is a type of a STN type liquid crystal display, so-called blue mode display, which turns blue when no voltage is applied and turns white when the ON-voltage is applied. In this specification, when the expression “the voltage is applied” is used without special explanation, it refers to that the ON-voltage is applied.
For example, a polarization axis at the light incident side is arranged by rotating 15 to 45 degrees clockwise from an orientation at the light output side, and a polarization axis at the light output side is arranged by rotating 25 to 55 degrees clockwise from an orientation at the light output side. When no voltage is applied, for example, light in the red region near 620 nm wavelength is cut off and transmission light is viewed as blue. Contrast is low because light leaks even if it is cut off.
Japanese Laid-Open Patent No. 2004-62021 discloses a structure for improving a light blocking property at the time of cutting off state by having dichroic dye in liquid crystal formation of a blue mode STN type liquid crystal display. As other means for improving a light blocking property, an optical compensator can be used.
Normally in the blue mode, a white back light is used as a back light; however, a monochromatic light source such as a light emitting diode (LED) may be used. In this case, by decreasing a transmission rate and improving a light blocking property when no voltage or non-selection voltage is applied at light emitting wavelengths of the back light in order to increasing the transmission rate and improving a light transmitting property when the ON voltage is applied, a high contrast normally black display whose display color is a color of the back light can be obtained.
In case of a mode wherein monochromatic is displayed by the normally black display, it is required to improve contrast between a state when no voltage or non-selection voltage is applied and a state when the ON-voltage is applied. Moreover, in case of a liquid crystal display mounted on a car, improvement in viewing angle properties and thermal properties are desired.